1. Field of the Invention
This invention relates to power supplies and, more particularly, to power supplies of the switching converter type using magnetic amplifier or saturable reactor voltage regulating means.
2. Brief Description of the Prior Art
Switching power supplies are frequently used to provide a plurality of separate outputs, and independent control of the outputs is often required. This has usually been done in the past by packaging multiple power stages in one unit and using a separate control loop for each output. A separate switching power stage and transformer are needed for each controlled output. This solution works well but is expensive.
Another solution, involving the use of a magnetic amplifier, which is described in the article "Switch Mode Converter Using High-Frequency Magnetic Amplifier" by Hiramatsu, Harada and Ninomiya appearing in Power Conversion International for March-April 1980 at pages 75-82, allows control on the secondary side of the transformer. Thus, one transformer delivers multiple, independently controlled, outputs with large cost and size savings.
As will be explained more fully below in connection with FIG. 1, prior art converters of this type use a switch transistor to drive the primary of a transformer which has two output windings. One of the output windings feeds a rectifier and filter to supply a first output voltage which is sensed by a control circuit which adjusts the operating duty cycle of the switch to maintain the output voltage at a preset value. The second output winding is connected to a rectifier and filter through a saturable reactor having a core with a square B-H loop. When the core is not saturated, the reactor exhibits a high impedance and prevents the voltage in the second output winding from reaching the rectifier and filter. The voltage will cause the core to saturate after a period of time determined by the starting point on the B-H loop and the applied voltage. When the core saturates, the reactor switches to a low impedance value permitting the voltage to be applied to the rectifier and filter. Between pulses, a control circuit forces the reactor to reset with a current which is poled in a direction opposite to the direction of the current during the active conduction period. The reset point is adjusted in response to the output voltage to maintain the output voltage at a preset value.
While this scheme works well, there are cost problems associated with the construction of the saturable reactor. A square loop material with a high degree of squareness is required. This is usually obtained by using metal tape-wound cores of permalloy which are expensive. Square loop ferrites may also be used, but the available ferrite materials are quite lossy, leading to heat problems. To maintain squareness, an ungapped magnetic structure, usually a toroidal core, is required. These are expensive to wind and difficult to mount.
It has been suggested in U.S. Pat. No. 2,753,518 that a magnetic amplifier voltage control system use relatively inexpensive substantially zero remanent, moderately low permeability cores with a B-H characteristic having poor squareness. In order to control the direct current power to a load, the reset point of the core is controlled by adjusting a current supplied during a control half-cycle in a control circuit including a variable resistance, a rectifier and a control winding for the saturable reactor.
It is also known from U.S. Pat. Nos. 2,054,496, 2,638,571 and 3,182,249 to control the current through a reactor by controlling the application of a short circuit across the reactor or across a winding coupled magnetically with the reactor.